Static build-up is a common problem in polymeric films and can lead to dust accumulation, decreased machinability (e.g., sheets sticking to each other in the dispenser for sheet-fed label devices, or problems with die cutting because of static interference in label applications), and sudden discharge when roll capacitance is exceeded. This presents not only processing challenges, but also concerns with respect to sudden and large static discharges. It is desirable to mitigate static build-up in films used in packaging for electronic equipment to avoid damage caused by electrostatic discharge.
One solution to such problems is to utilize multi-layer electrostatic dissipative structures, such as those described in U.S. Pat. No. 5,914,191. For example, film structures comprising a core layer including a polymer having a haze value less than 5% and one or more outer layer including of a blend of copolyester and polyetherurethane or polyaniline are known. WO 02/074534 discloses a multi-layer structure comprising at least one electrostatic dissipative outer layer, preferably comprising polyaniline and a conductive core layer. The surface resistivity of the multi-layer structure is less than the surface resistivity of the outer layer alone or in another multi-layer structure absent contact with the conductive core layer. WO 2009/135998 discloses an ungrounded type multilayer packaging comprising a nonconductive multilayer packaging material containing at least one polymeric thermoplastic dissipating layer. The dissipating layer comprises a polymer having polyethylene oxide and polyolefin blocks, thereby providing an electrostatic discharge energy attenuation of more than 40 dB.
Another way to control static in polymeric films is to modify a polymer's antistatic characteristics and dissipative capacity by increasing the rate at which the polymer can dissipate static charge, generally by increasing conductivity. This has been accomplished in the past by the use of various antistatic agents. Incorporation of antistatic agents may be accomplished in a number of ways: application of external antistatic agent as a coat or paint; incorporation of migratory antistatic agents into the external polymer layers; blending conductive fillers into non-conductive polymers; and incorporating inherently conductive polymers (sometimes referred to as “ICPs,” which generally have a surface resistivity of 101 to 106 Ohms/square) into the non-conductive polymer.
Also, it is known to employ internal antistatic agents which are volume dispersed by admixing in the polymer, i.e., incorporated into the polymer by compounding or extrusion prior to or during molding or film-forming operations. When used in this manner such internal antistatic agents work by migrating to the polymer surface and absorbing moisture from the air, thereby providing antistatic effect. This migration is colloquially referred to in the art of antistatic polymer technology as a “blooming” or “bleeding” effect. These migratory antistatic agents (e.g., fatty alcohols or esters), however, typically have low molecular weight, and thus are volatile, and tend to be less stable during extrusion. And there migration within the film often leads to undesirable consequences.
Thus, there remains a need for films having a desirable balance of properties that can dissipate charge while maintaining the integrity of the polymer matrix and reducing unwanted migration and/or degradation of the antistatic agent during processing. More particularly, because the surface layer of a film is often coated, there is a need for films where the antistatic agent is embedded or incorporated in the film for stability yet still capable of providing effective antistatic properties, even under a layer of coating.